Pattern to access to object instance from other object instances

Question

Thanks in advance. Given these example of classes...



    # registers all that happens during a combat
    class CombatLog:
        def __init__(self):
            self.actions = []

    # it handles a combat
    class Combat:
        def __init__(self):
            self.fighters = []   # list of fighter instances
            self.combat_log = CombatLog()
        def run_round(self):
            # do whatever
            self.combat_log.log(...)

    # are the fighters in a combat
    class Fighter:
        def __init__(self):
            self.property1 = 1
            self.property2 = 2
            self.attacks = []  # list of attack instances
        def attack(self):
            # do whatever
            self.combat_log.log(...)  # clean way to access the log?

    class Attack:
        def __init__(self):
            self.damage = 10
        def cast(self):
            # do whatever
            self.combat_log.log(...)  # clean way to access the log?


    # from somewhere else in the code
    combat1 = Combat()
    combat2 = Combat()

I am wondering what would be the best pattern to handle the next requirements:

There is one unique instance of the CombatLog per Combat. So if there are 2 Combat instances, the combat would have 2 CombatLog instances.
I want to use the combat log everywhere is needed. For example, if a Fighter, the Combat or Attack, needs to access to the log. The CombatLog is like some "global" object ONLY within that Combat and all the instances within that Combat.
When the Combat is done and removed from the memory, the CombatLog gets removed as well.

The approaches I am thinking of, but I am not proud/sure of them:

Pass the log instance through all the children instances (the fighters, and the attacks)
Make some kind of log manager (like Singleton), and then request the log for that combat from where I want. But i have a similar problem, which is that I have to pass the id of that combat to all chindren in order to request the proper log to the log manager... Which is more o less the same problem than before.
I was thinking to make some kind of dependency injector, like using decorators or so, but I always crash against the same problem (the one before).

Maybe I need another approach, but I can't think of... The code is in Python.

Thanks a lot for your time.

score 1 · Accepted Answer · answered Jan 10 '15 at 22:38

1

If a fighter exists only in a combat, let it know in which combat it is:

class Fighter:
    def __init__(self, combat, name):
        self.name = name
        self.combat = combat
    def attack(self):
        self.combat.log.append('{name} attacking'.format(name=self.name))

If a fighter can switch combats, add a combat setter:

class Figthter:
    # init(self, name) ...

    def set_combat(combat):
        self.combat = combat

answered Jan 10 '15 at 22:38

Andrew_Lvov

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thanks a lot for your time. Yes this is the first approach I did. What I don't like, is that all that is created within that combat (fighters, attacks, teams, etc...) I have to give the combat object in all constructors... This is what I was not sure if would be a more "automated way" to do this. – timmyn Jan 11 '15 at 11:58
At the end I decided to approach this way. Thanks! – timmyn Jan 11 '15 at 14:20

score 0 · Answer 2 · edited May 23 '17 at 10:25

0

There is another simple approach, anyway I think that the best option would be to have a pointer inside the other objects refering to the object Combat, but this doesn't seem possible:
Pointers in Python?
You could maintain a dictionary of Combats and pass their key to themselves and to the other objects:

combats = {uuid0: Combat(), uuid1: Combat()}

And access them like

class Attack:
    def __init__(self, combatUUID = None):
        self.damage = 10
        if combatUUID: self.combatUUID = combatUUID 
    def cast(self):
        # do whatever
        if self.combatUUID in combats: combats[self.combatUUID].log(...)

edited May 23 '17 at 10:25

Community

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1

answered Jan 10 '15 at 22:37

TuTTe

164
4

You can certainly maintain a reference to `Combat` instances within other objects. No problem there. – jme Jan 10 '15 at 23:02
thanks a lot!. Yes this is the second approach I though, but like I said to @Andrew_Lvov, all that is created within that combat (fighters, attacks, teams, etc...) I have to give the combat uuid in all constructors... So I don't know if can be more automatic way to do this – timmyn Jan 11 '15 at 12:01

Pattern to access to object instance from other object instances

2 Answers2